Electrical systems often include components in different locations. Each component may operate relative to a reference voltage such as a circuit ground, building ground or earth ground. If the reference voltage is different between components, the components may not be able to interact properly. In a measurement system, measurements may be performed by sensors at different locations, e.g., temperature measurements performed by thermocouples in different parts of a factory. Each sensor may output its data to a respective channel, and all the channels may be multiplexed to a measuring device located elsewhere in the factory, such that only one selected channel is output to the measuring device at any particular time. As a result of capacitive, inductive or electromagnetic coupling, each channel may include a common mode voltage (CMV) in addition to voltages corresponding to sensor data. In the case of thermocouples, the input may be on the order of 10 μVs, whereas other sensors may provide a voltage input between +10 V and −10 V that sits on top of the CMV. In comparison, the CMV can be quite large relative to the input signal. The CMV may vary between channels and may also be different from the voltage of a ground signal used by the measuring device. To ensure proper operation of the measuring device, the ground signal is typically adjusted towards the CMV of the selected channel. A supply voltage to the measuring device may be generated with reference to the ground signal. Therefore, when the ground signal is near CMV, the supply voltage is also near the CMV, i.e., at a voltage level suitable for driving the measuring device when the input to the measuring device includes the same CMV.
One method for connecting channels to the measuring device is to use a PhotoMOS as a switch for each channel. A photodiode within the PhotoMOS generates a gate-to-source voltage (Vgs) to turn on the PhotoMOS, thereby connecting the output of the sensor to the input of the measuring device. The Vgs is generated relative to the CMV of the channel to which the PhotoMOS is connected. PhotoMOS are relatively large devices in part because of the sizes of the photodiodes and associated hardware (e.g., a light-emitting diode to activate the photodiode). PhotoMOS also need to be controlled (turned on or off) using respective control lines in a one-to-one manner Therefore, PhotoMOS may not be suitable for implementing a multiplexer when there are many channels or when there are space constraints. Accordingly, a need exists for improved ways to multiplex channels that have different CMVs.